1. Field of the Invention
This invention relates to the valving means for instrument installations when flow measurement and/or control is desired in a fluid flow line using the differential pressure method for determining the flow rate. More particularly, the present invention concerns a rotary spool type instrument mounting and mode selecting manifold having a five-valve-function being "RUN", "ZERO--Lockup", "OFF--Lockup", "VENT" and "PURGE" and permitting simple and efficient calibration of a differential pressure transmitter instrument that is mounted thereto. Even more specifically, the present invention concerns a mounting and mode selection manifold providing instrument personnel with the capability of achieving "run", "zero" and "off/calibrate" functional modes simply by rotational settings of a rotary spool member of the manifold.
2. Description of the Prior Art
The differential pressure method of flow determination of such fluids as liquids, natural gas or other gases, and the like, is widely accepted in industry as an accurate means of flow rate determination. Differential pressure transmitters to sense and transmit the pressure differential from the pressure taps of the primary element (orifice plate, flow nozzle, venturi and the like), are well known in the art and are prized for their fast speed of measurement response particularly with all-electric instrument systems. These differential pressure transmitters are normally remotely located from the computer or meter location which processes the differential signal into a flow rate or flow volume over time.
The fluid flowing in a flow line or "process" is in pressure communication with the differential pressure transmitter through conduits from the pressure taps in the primary element to the transmitter.
To routinely check the transmitter for accuracy, a means to equalize the high and low pressure signals at the transmitter must be provided to show that zero pressure differential (no flow) results in a zero signal output from the transmitter. Checking zero results from blocking one or both of the signal lines from the primary element to the transmitter and opening an equalizing line in the manifold causing a zero pressure differential at the transmitter.
The present state-of-the-art instrument transmitter manifold used for natural gas measurement contains five (5) valves which must be manually manipulated to obtain the desired zeroing results. This manual manipulation procedure is time consuming and costly for the operator who must operate these valves in a specific sequence to obtain the zero condition and then return the manifold to the operating service in the specific reverse sequence. If the zero and return sequences are not precisely followed, the sensitive differential pressure transmitter can be damaged.
For the measurement of high volumes of expensive natural gas, it is mandatory that the most accurate differential pressure measurement be made and the system design be such that all potential errors be eliminated or minimized.
It has been well established by both field measurement research and field documentation that improved measurement accuracy results if the conduit from the primary element (orifice plate, etc.) to the transmitter has the same diameter as that of the primary element (3/8" for orifice flange unions through 12" diameter) and has no expansion/contraction points in the differential pressure conduits such as presently experienced with rising stem plug or gate valve seat cavities. Improved measurement accuracy also results when the transmitter is connected in close coupled relation with the primary element.
In the "run" position, if the high differential pressure passage used to check zero were to leak from the high differential side to the low differential side, the resulting flow measurement would create an error and a loss of revenue for the seller. To prevent this possibility, the equalizer passage of current 5-valve manifolds have two closed needle valves with an open bleed-to-atmosphere valve (normally open) between the two closed needle valves. If either needle block valve leaks, the leaked fluid will be vented to atmosphere through the open bleed valve and not affect the measurement. The manifold of the present invention retains this measurement safety feature but eliminates the needle valves with "O-ring valves" which need not ever be manipulated. If the O-rings were to leak, the leaked fluid will vent to atmosphere in the same manner and will not affect measurement accuracy.
Differential pressure transmitters are subject to "zero shift" (a measurement error if not corrected) when uncontrolled pressure waves in the pipeline travel from upstream to downstream. The intensity of these pressure waves is much larger than the fluid differential pressure being measured and reach the orifice plate (or other differential primary device) on the high side first. So long as the pressure wave is less than preestablished during transmitter calibration, there will be no zero shift. However, when the installation is first commissioned, (that is, atmospheric pressure is in the transmitter chambers) present manifold design dictates that the equalizer line be open and the transmitter be pressurized by opening the high side (up stream side) valve first. The rotary spool type manifold represented by this invention has a built-in feature that allows the high side to be pressurized before the low side is pressurized establishing that zero shift from pressure overage is eliminated or minimized.
Many companies check the zero and span calibration of the transmitter at the measurement location and recalibrate in-place if required. Recalibration requires the use of sensitive, easily damaged special differential pressure calibration equipment. To protect the calibration equipment using presently marketed manifolds, the field technician must be certain through his valve manipulation procedure, that full line pressure from the process does not reach the sensitive calibration equipment, thus averting its damage. The manifold of this invention has a built-in safety feature requiring no valve(s) manipulation and protects the transmitter from being subjected to the process pressure while calibration is in progress. This safety feature further blocks the process pressure if the transmitter is removed from the assembly thereby eliminating discharge of process fluid to the atmosphere and potentially harming the technician and/or the environment. This safety feature may be released only by conscious manual effort to place the manifold in the active measurement mode ("RUN" position).
As natural gas usage increases, so do the pipe sizes increase that transport the gas. Therefore the diameter of the orifice plate increases to accommodate the larger pipe sizes. In line sizes 6" diameter or greater, the impingement of the high velocity gas on the orifice plate bows the standard 1/8" thick plate causing an error in measurement. This problem is solved by increasing the orifice plate thickness from 1/8" to 1/4". Since it is troublesome and expensive for the field technician to have two orifice plate thicknesses depending on the line size, many companies use 1/4" thick orifice plates for all line sizes.
When using 1/8" thick orifice plates, the center-line spacing between orifice flange union signal taps is 2.125". When using 1/4" thick orifice plates, this dimension increases by 1/8" to 2.250. This means that there is a 1/16" offset between the 3/8" signal passage diameter of the supporting flange-to-manifold adaptor (commonly called "the FUTBOL") and the manifold inlet port on each the high and low pressure signal passages. Since it is desirable to provide a manifold having a uniform 3/8" diameter signal path from the orifice flange union through the manifold and into the transmitter, this 1/16" offset creates a point reduction of 3/8" to 5/16", thus violating the design requirement of 3/8" signal passages with no interruptions which may lower the accuracy of measurement. It is desirable therefore, to provide a rotary spool type mode selection manifold which provides a uniform 3/8" diameter signal path from the orifice flange union through the manifold and into the transmitter that is mounted to the manifold body and which accommodates the 1/16" offset that exists when 1/4" orifice plates are used.